My HMDT, Tooms, has been showing some tail biting (I think?) since shortly after I got him a little over a month ago. He is in a divided, cycled 5g that gets 50% water changes twice a week. I use stress coat to condition and always put a little extra in, and his water is at a constant 76F - I know it could be a little warmer, but for right now all I have is a preset heater. Basically, it's been getting worse lately, and I just want to make sure it's just tail biting and not fin rot. My other male (CT) in the other half of the tank has had consistent new growth on his fins, so I'm pretty sure it's not rot (unless it is, but just isn't contagious) but just need to check.

Here are a few pics in chronological order from when I first got him to now:

He does have a couple ghost shrimp in his side with him (as seen in the last, most recent photo), which might be stressing him out although they leave him alone and he just steals their food, lol. I have nowhere else to put the shrimp because my other male and my females in my sorority will kill them, and I kind of like them otherwise I'd get rid of them. When they die I won't be replacing them.

It could be biting but it looks like his overall condition is deteriorating along with his fins. Do you see any darkening to the tops of the fins.. brown or red or are they looking thinner there? Or just chunks missing?

Housing
What size is your tank? 5g
What temperature is your tank? 76F
Does your tank have a filter? yes, tetra whisper
Does your tank have an air stone or other type of aeration? no
Is your tank heated? yes
What tank mates does your betta fish live with? His tank is divided with a CT male on the other side. 4 small ghost shrimp live on the same side as this fish.

Food
What type of food do you feed your betta fish? He eats mainly live daphnia with the occasional pellet, frozen blood worms or frozen brine shrimp if daphnia aren't available. He has also been eating algae wafers and flakes because he steals from the shrimp.
How often do you feed your betta fish? once or twice daily, depending on how fat he looks. Lol, hate to say it but he is a pig and because he steals from the shrimp he tends to look bloated pretty quickly. Actually kind of surprised you said he looks like he's lost weight...must just be the angle of the photos.

Maintenance
How often do you perform a water change? Twice weekly
What percentage of the water do you change when you perform a water change? 50%
What type of additives do you add to the water when you perform a water change? Stress Coat

Water Parameters:
Have you tested your water? If so, what are the following parameters?

Symptoms and Treatment
How has your betta fish's appearance changed? Tail receding, mostly just the bottom part
How has your betta fish's behavior changed? No
When did you start noticing the symptoms? About a week after I got him
Have you started treating your fish? If so, how? Not sure if extra stress coat counts.
Does your fish have any history of being ill? Not that I know of.
How old is your fish (approximately)? I've had him since November 18, he's pretty small but no idea how old he is.

His fins look pretty normal, except for the obvious chunks taken out of them. He has natural red coloration on his fins so that is all I'm really seeing.

The last picture looks like he has a little rot developing, could be he is biting and it got infected causing rot too start.

I agree that he looks like he's lost weight, could be due to the fact that you are feeding mainly daphnia which actually can act as a natural laxative for bettas. I would stop the daphnia as a main source of food and switch to high quality pellets like NLS or Omega One

I have Omega One, so I'll start feeding him that. I know daphnia are good for the digestive tract, but they are also one of the fishes natural foods and are comprised of 50% protein and 25% fat, which is better than the pellets at 46% protein and 5% fat. I would think feeding solely live daphnia would be the best thing for him! Regardless, my daphnia cultures at work crashed so I won't be able to bring any home for a few weeks, so Omega One will be his main source of food now.

If he is developing a bit of rot, would putting him in a QT tank be my best bet? Or will he heal with clean water/stress coat in his current tank?

Water practices look pretty good. He's on the very tail end of tolerable on temps. He would be a lot happier if you bumped his temp up 2-3 degrees. 78-80ish is really ideal.

What does his poop look like? Feeding live food leaves a risk of developing internal parasite infections which could also cause the weight loss.. you can really tell by his side that he's no longer as round/filled out. His stomach may look larger now when he has a big meal because the rest of him is smaller. Sometimes fish also bite their tails along with the infection. Feeding daphnia occasionally help aid in digestion is fine. Feeding a blood worm for a treat like candy once a week is also fine. Brine shrimp are great but shouldn't be fed exclusively either. I would also switch to feeding a pellet as your staple and supplementing with the other things you are using. I also like NLS the most but omega one is okay. You can feed 3-4 daily split up as a staple except on the days you choose to supplement with brine shrimp or a blood worm. I'd save daphnia for maybe just the day fast day.. feed that instead of a straight fast maybe. Daphnia and blood worms are really just treat foods. Even brine shrimp don't have all the essential nutrients.

I would QT and treat with AQ at 1 tsp per gallon predissolved for up to 10 days or until you see the fins stop receeding and start growing a little. Then you know you're in the clear. Increased water changes in the QT, but make sure you can keep him warm in the QT. Don't let it fall to room temp.

Tooms has nice brown healthy poops. The daphnia that he is fed are cultured for water toxicity testing and are probably the healthiest live food he could ever eat. I will say that we also culture fish for testing and they eat solely brine shrimp. Right after they hatch they are fed newly hatched brine shrimp and then graduate to adult frozen brine shrimp for the rest of their lives. They are fat, healthy, and breed like rabbits. Do bettas have much different nutritional requirements from other fish? I guess from a biological standpoint, natural foods just seem better than feeding them some processed pellets. My CT has definitely gained weight from eating daphnia, when I first got him he did not eat anything else (no pellets, nothing) and is nice and healthy and like I said previously has fantastic fin growth. Tooms also looks the same as he does in that first picture(nice and fat), but for some reason the more recent photos don't look like that.

I have a hydor heater for bettas and bowls that kept my 2g at about 80 when my CT was housed there before I got the 5g. I will put Tooms in the 2g and start the salt treatment. I am planning on getting an adjustable heater so I can heat the 5g to about 80 also, just haven't gotten around to it yet. Thanks!